Fusing Equipment
Fuse Links
Selection based on lightning Surge Probability
Reprinted from May 28, 1962 issue of ELECTRICAL WORLD
Copyright 1962, McGraw-Hill Publishing Co.. Inc.
All rights reserved.
Reference Data
R240-30-5
Page 1
Replaces 240-30BC-1
Fuse Links for Carrying Lightning Surges
R. H. AMUNDSON. Senior Engineer,
Systems Protection. McGraw-Edison
Power Systems Division, South
Milwaukee, Wis.
The accompanying diagram is
aimed at aiding designers in those
applications on distribution systems
where fuse cutouts are located on
the source side of lightning arresters.
With this arrangement lightning
surge current discharged by the
arrester passes through the cutout
and associated fuse link and may
damage or blow the link if it is too
small.
Thus the selection of link sizes
for these applications generally is
a compromise. While large links
will blow less frequently from
lightning, a smaller link may be
dictated by overcurrent protective
requirements or coordination considerations. Selection of the best
link size can be made with greater
certainty when based on the probability of links being damaged by
lightning.
Show Probabilities
f u s e Link Rating, Amperes
Recently developed curves relate PROBABILITY that lightning surges will damage links located on the source side
the surge resistance of links and the of arresters. The plot is for rural areas with an isokeraunic level of 30. Probprobabilities of lightning strokes of abilities for other isokeraunic levels can be plotted in direct proportion to 30.
various magnitudes. With these The curves are also valid for urban circuits
curves the relative performance of
various link sizes and types of links
may be compared and the probable ties, the probability of a link being K links. The curves also show that
performance of a particular choice damaged then was determined. For extremely large links are not necesexample, a 17,000-amp surge is sary for good performance. Very
may be determined.
The curves were developed by required to damage a 15K link and little improvement is gained by usfirst determining the magnitude of such a surge occurs 0.018 times per ing iinks larger than 25 amp.
The accompanying curves are for
arrester per year. For every 100
10 X 20-surge current required to
damage various sizes and types of links and arresters installed, then, an isokeraunic level of 30 in rural
fuse links. This is done readily by 1.8 links, would be damaged. This areas. For different isokeraunic
comparing the heating effect of the percentage determines one point on levels the curves can be corrected
surge current (as measured by the a curve. Similar calculations for to the reference level of 30 in direct
other link ratings establish sufficient proportion to the ratio of storm
integral of i2dt) with the i 2 t necespoints to complete the curves.
frequency. For link performance on
sary to damage the link.
by
installurban circuits, the curves give a
The
advantage
gained
The next step was to determine
the frequency of a surge of such ing T links instead of K is revealed larger margin of safety because
magnitude from published probabil- by the curves. Size for size, T urban circuits are less exposed to
ity curves. By relating these quanti- links will blow only half as often as lightning than are rural circuits.
December 1968